Complementary to the Fourier space measures are real space topological tools which can be directly employed on 21cm images from upcoming interferometers such as the Square Kilometre Array (SKA)[1]} which would enable direct imaging from the EoR [2]} (in addition to better constraints on power spectrum). The advantage of imaging is that it contains both phase and amplitude information of a field. Since information from all orders of correlation are contained in an image, it encapsulates the non-Gaussian information which is otherwise missing in the power spectrum. There are methods which rely on studying the size statistics of ionized bubbles in tomographic images [3]}, [4]} or based on mathematical morphology [5]}, [6]}, [7]}. However, these methods rely on first identifying or defining an ionized bubble in a 21cm map which can be difficult in noisy 21cm images [8]}. There are methods based on percolation theory [9]}, [10]}, [11]} and persistent topology [12]} which have been used to theoretically study the topological phases of ionized bubbles during EoR.
There are also methods based on Scalar Minkowski Functionals (SMFs)  [13]}, [14]}, [15]}, [16]}, [17]}, [18]}, [19]} for ionization field or the 21cm brightness temperature field during EoR. These methods can be used to study the topology of an entire field and its systematic variation with field threshold, which can give insights useful for discriminating different EoR scenarios.
